Sherlock series 4: First picture of Martin Freeman and Benedict Cumberbatch hints at time travel

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The BBC has dropped a surprise new picture from the Sherlock special, showing lead stars Benedict Cumberbatch and Martin Freeman in intriguing old-fashioned get up.

Executive producer Sue Vertue first tweeted an image of the new draft script by Mark Gatiss and Steven Moffat, dated 22 November, before following up with a promised photo of Sherlock and John Watson (see above).

The accompanying hashtag, #221back, is a reference to Sherlock’s address of 221B Baker Street. It first appeared in July, when the BBC announced at 2.21pm that the detective drama would be returning in 2015, as previously hinted at by Freeman.

Fans have been told to expect “the most shocking and surprising series of Sherlock yet”, but have to wait until next Christmas for the special and a resulting 2016 fourth season.

“Of course it’s far too early to say what’s coming but we’re reasonably confident that the very next thing to happen to Sherlock and John is the very last thing you’d expect,” said Moffat earlier this year.

Last month, Gatiss added that upcoming episodes will be "devastating". "Expect tragedy as well as adventure, that's just how it goes," he told Radio Times, before noting that the show's plotline will not necessarily follow that of Sir Arthur Conan Doyle's original books.